Digital signal processor (DSPs) are used in digital mobile cellular radio systems to enable echo cancellation, speech encoder/decoder (codec) functions and channel encoder/decoder functions for TDMA channels. These digital signal processors perform in some installations echo cancellation on the switch side of the cellular infrastructure. Some equipment manufacturers provide for these functions on the cell site end of the cellular system. In both cases, the speech and channel encoder/decoder (codec) designs and implementations are made to comply with Communications Technology Industry Association's (CTIA) Digital Cellular Interim Standard (IS-54) specification.
In a TDMA mobile-to-land connection, audio signals arriving at the digital signal processor from the land source is first subject to echo cancellation. The echo cancelled signal is then compressed into IS-54 VSELP signal format with a 8:1 ratio and the compressed signal is channel encoded and sent over the air to the mobile. VSELP signal arriving at the mobile is channel decoded and then used to reconstruct the original audio signal by the speech decoder. The entire speech encoding and decoding process introduces perceptible coding noise to the reconstructed speech signal. Audio signal travelling in the reverse direction (mobile to land) is subject to the same encoding and decoding processes but with no echo cancellation by the digital signal processors.
A TDMA mobile-to-mobile connection is realized basically by connecting two TDMA mobile-to-land calls back to back between two digital signal processors. Thus, an audio signal travelling in either direction is subject to two speech encoding and decoding processes operating in tandem. Thus, the coding noise introduced at each end degrades the quality of voice signal received by each subscriber.
Accordingly, a requirement exists to reduce the amount of coding noise which is generated during mobile-to-mobile communication.